Various systems have already been proposed for automatically adapting the mean sound level of radio receivers, in particular of radio broadcast receivers located on-board motor vehicles, specifically for the purpose of adapting the sound volume to the ambient noise level. Those systems act by responding to noise sources that are external to the receiver.
With television receivers, whether monochrome or color, monophonic or stereophonic, and in particular with domestic receivers, the problem for viewer comfort is generally not associated with noise from the surroundings, particularly since it is possible to eliminate such noise, e.g. by wearing headphones. contrast, the viewer generally finds that on changing channel, the mean sound level usually varies, which means that the sound volume must be adapted on each occasion.
Even with a remote control, it is disagreeable for a viewer who is "channel punching", i.e. skipping frequently from one program to another, to have to adapt the mean sound level manually by means of the manual control for adjusting volume.
Even more difficult is the observation that for any one channel selected by the viewer, the mean sound level often varies over a wide range. Thus, when broadcasting advertising sequences or signature tunes at the beginning or the end of broadcasting, the mean sound level is generally higher than within a normal program, such as a broadcast based on speech. Musical programs can also give rise to highly contrasted sound levels, generally greater than those of broadcasts based on conversation or on reporting.
There is thus a need for a system enabling the sound volume of broadcast programs to be equalized automatically.
Unfortunately, in practice, there is a difficulty associated with the fact that the sound is not transmitted completely independently of the image. Thus any action taken on the audio path has repercussions to a greater or lesser extent on image quality, particularly if synchronization signals are degraded, and conversely image processing does not provide a way of solving problems associated with sound quality.
Thus, the automatic gain control (AGC) systems of television receivers act only on the image itself, and do not act on the audio paths to provide an equalization function.
Also, the number of existing television receivers is very large and it would be desirable for means for automatically equalizing sound volume to be capable of being applied to receivers that are already in operation and that do not have such means.